The invention relates generally to balloon angioplasty for dilating obstructed blood vessels and more particularly, to a system for controlling the heat applied by a dilatation balloon in thermal angioplasty.
In a stenosis caused by plaque creating an obstruction in a vessel wall, a dilatation balloon catheter may be used to enlarge a passage through the stenosis. Typically, the balloon is located at the stenosis and pressurized liquid is passed down the catheter to inflate the balloon, causing it to expand and thereby compress the plaque against the artery walls. Inflation of the balloon results in expansion of the narrowed lumen in the artery, thereby increasing the blood flow after the balloon has been removed. Such outward compression results in stress on the plaque, sometimes causing cracking, tearing and stretching. In some cases, after the balloon catheter is removed, torn plaque and tissue become dislodged from the vessel wall resulting in abrupt reclosure of the vessel. Even when abrupt reclosure does not occur, it is thought that the irregular inner surface of the vessel wall may contribute to restenosis at the same location.
One approach to promote the healing of blood vessels damaged by balloon angioplasty has been the application of heat during the angioplasty procedure. During balloon inflation, the disrupted tissues of the plaque and the arterial wall are heated in order to fuse together fragmented segments of tissue and to coagulate blood trapped within dissected planes of tissue. Upon subsequent balloon deflation, a smooth, cylindrically-shaped channel results.
In one dilatation system where the balloon is filled with liquid for balloon inflation, a heating element, such as a coil, is located on the catheter in the balloon and heats the inflation liquid which transfers the heat to the walls of the balloon by conduction. The balloon in turn transfers the heat to the disrupted plaque and tissues. An electrical power supply provides electrical power to the heating element for the generation of heat. In some prior devices, a radio frequency (RF) generator provides the electrical power to the balloon heating element to generate the heat. A temperature sensing device or devices are located on the heating element and a feedback loop is established with the signal from the sensing device used to control the RF generator to achieve the desired temperature.
In a typical automatic control system, a sensed temperature is compared to the temperature set by the operator and an error signal provided. The error signal is used to automatically regulate the electrical power supply to provide more or less RF energy to the heating coil to obtain the desired temperature. If only a single sensor were used to generate the error signal, inaccuracy of or damage to the sensor may result in over-heating the balloon. If the sensor were to become inaccurate in such a way that the error signal indicated a much lower than actual temperature at the heating element, the RF generator may continue to apply power to the heating element and the balloon may become overheated. Overheating the balloon such that the boiling point is reached and gas and steam are formed in the patient's vessel could have serious effects on the surrounding tissue. Thus, an RF generator which provides multiple automatic controls over the generation of heat would be desirable in a more fail safe system.
Another concern with heat dilatation systems is the inadvertent application of electrical power to the heating element before it is properly positioned at the site to be heated. For example, in the case where the RF generator includes an "RF ON" switch which has been inadvertently left in the "ON" position, and the RF generator is turned on before the dilatation balloon is properly positioned, the heating element may immediately begin to apply heat. Such an inappropriate application of heat may cause injury. Thus, it would be desirable to provide an RF generator having features which would guard against such an inadvertent application of heat.
An additional consideration in thermal dilatation is exposure of the patient to electrical shock. Because the heating coil and the sensors are all used invasively, the patient could be directly exposed to electrical shock in the event that any sensor, or the heating coil itself, is exposed to the patient's bloodstream. For example, should the dilatation balloon burst during the dilatation procedure, the sensors and the heating coil would be exposed to the bloodstream. Although some prior systems include isolation from earth ground, such dilatation systems are sometimes mounted on a metallic pole next to the patient's bed. In the event that exposure of the patient to an invasive electrical lead occurs and a grounding malfunction occurs inside the system, the pole could function as a conductor to earth ground and the patient could experience an electrical shock. Isolation of the patient from exposure to such electrical shocks would be desirable.
A further consideration in the use of sensors to accurately determine a temperature is the thermocouple effect resulting from physically connecting the electrical leads of the sensor to the electrical leads of the electrical power supply when the sets of leads are formed of dissimilar materials. In the case where the materials of the two pairs of electrical leads are incompatible, the physical contact of those leads at a connector can itself generate an electrical current which will vary in dependence on the temperature. This current will add to the electrical current generated by the sensor thus making the electrical current obtained from those leads inaccurate for use in determining the temperature sensed by the sensor.
Hence, those concerned with heat generation and heat control in thermal balloon dilatation systems have long recognized the need for a system which can more quickly and more reliably detect a sensor inaccuracy or other heat control inaccuracy which may lead to over-heating, and which can take appropriate action to protect the patient. It would also be beneficial to provide greater control over the application of electrical power to the heating element to avoid inadvertent applications of heat, as well as to provide a system which lowers the risk of electrical shock to a patient in the event of exposure to the electrical leads of invasive sensors and heating coils. The present invention fulfills these needs.